Mass spectrometer ion collector with a secondary particle collecting electrode extending over the collector



June 29, 1965 B. N. GREEN 3,192,383

MASS SPECTROMETER ION COLLECTOR WITH A SECONDARY PARTICLE COLLECTINGELECTRODE EXTENDINGVOVER THE COLLECTOR Filed Dec. 10. 1962 PRIORART a meUnited States Patent 3,192,383 MASS SPECTROMETER ION COLLECTOR WITH ASECONDARY PARTICLE COLLECTING ELEC- TRODE EXTENDING OVER THE COLLECTORBrian Noel Green, Urmston, Manchester, England, as-

signor to Associated Electrical Industries Limited, London, England, aBritish company Filed Dec. 10,1962, Ser. No. 243,536 Claims priority,application Great Britain, Dec. 22, 1961, 45,997/ 61 2 Claims. (Cl.250-413) The present invention relates to mass spectrometers and inparticular to apparatus for collecting ions after they have passedthrough the magnetic analyser of a mass spectrometer and means forsuppressing adverse effects due to secondary emission in the collectingapparatus.

In a mass spectrometer primary ions are produced from a sample to beanalysed and are formed into a beam and projected into a magnetic fieldformed between the poles of a magnet in the magnetic analyser. Each ionis deflected on passage through this magnetic field in accordance withits mass/charge ratio and if the sample produces ions of differentmasses the beam of ions will be dispersed into a spectrum.

In order to detect ions of different mass/charge ratio it is convenientto use collecting apparatus including a sensitive plate to receive thedispersed beam. Ions of different mass/charge ratios strike this plateand form marks at different positions on the plate. By subsequentexamination of the sensitive plate, the number of ions of any mass/charge ratio which strike the plate can be determined.

When a primary ion strikes the sensitive plate several secondary effectsoccur. Ions, neutral particles, photons and electrons may be emittedfrom the plate and there is a buildup of electric charge on the surfaceof the plate. The neutral particles and the photons travel in straightlines in random directions and the ions travel in curved paths and atleast some of these will eventually strike the magnet poles wheretertiary electrons will be emitted.

These tertiary electrons, under the combined actions of the magneticfield and the electric field produced between the sensitive plate andthe magnet poles, which are conveniently grounded, travel towards thesensitive plate and strike this plate at positions different from thatat which the primary ion struck.

This results in additional marks being formed on the sensitive plate inthe vicinity of the mark formed by the primary ion and these additionalmarks may totally or partially mask marks produced by other primaryions. This effect particularly occurs when a primary ion beam of highintensity is received by the sensitive plate. As a result thesensitivity of the mass spectrometer to detect primary ion beams of lowintensity in the region of a primary ion beam of high intensity isappreciably reduced.

The object of the present invention is to provide an improved form ofmass spectrometer ion collecting apparatus which tends to avoid theabove disadvantages.

According to the present invention a mass spectrometer ion collectingapparatus comprises a collector member having a surface coated with alayer of material sensitive to ions, an electrode extending over saidlayer and spaced therefrom, means for applying a positive potential tosaid electrode so that it attracts negatively charged secondaryparticles emitted from said sensitive layer when primary ions strikesaid layer, and a slit extending along said electrode to allow thepassage of primary ions on to said sensitive layer.

When the collecting apparatus is used in combination with the magneticanalyser of a mass spectrometer, the electrode prevents most of thesecondary particles emitted from the sensitive surface by incidentprimary ions from 3,192,383 Patented June 29, 1965 "ice striking thesurfaces of the magnetic analyser magnet poles and attracts any tertiaryelectrons emitted from the electrode itself or from the surfaces of-themagnet poles.

In order that the invention may be more readily understood referencewill now be made to the accompanying drawing in which:

FIG. 1 is an end view, in section, of part of the magnet analyser of amass spectrometer;

FIG. 2 is an end view of the magnetic analyser illustrated in FIG. 1including collecting apparatus in accordance with the invention; and

FIG. 3 is a side view sectioned on the plane III-III of the magneticanalyser illustrated in FIG. 2.

With reference to FIG. 1 the magnetic analyser comprises two magnetpoles 1, 2 defining a gap 3 through which a beam 4 of primary ionspasses. The ions in the beam are deflected in accordance with theirmass/ charge ratio in the manner described above as they pass throughthe magnetic field. The magnetic analyser is used in combination withcollecting apparatus comprising a plate 5 supported on insulatingsupports 11 from the poles and coated with a layer 6 of material whichis sensitive to ions. This material may be photosensitive emulsion.

When the primary ions strike the free surface of the layer 6 a positivecharge is formed on this surface and ions, neutral particles, photonsand electrons may be emitted from the surface. The neutral particles andphotons travel in straight lines in random directions, and

at least some will strike the top surface of one of the magnet poles.Some of the emitted ions will also travel in curved paths to strike thesurface of one of the magnet poles. Tertiary electrons may then beemitted from the magnet poles and these tertiary electrons will comeunder the combined effects of the magnetic field produced by the magnetpoles and the electric field produced between the charged surface of thelayer 6 and the magnet poles which are conveniently at ground potential.

The lines of force of the magnetic field extend in approximatelycircular paths as shown While the lines of force of the electric fieldextend directly between the layer 6 and the poles 1, 2. The tertiaryelectrons will therefore travel in helical paths towards the layer 6 andwill eventually strike the layer 6 producing additionalmarks on thesensitive layer.

FIGS. 2, 3 illustrate a modified magnetic analyser incorporatingcollecting apparatus in accordance with the invention. An electrode 7 issupported between the layer 6 on supports 11 and the poles 1, 2 andextends along the full length of the layer 6 substantially parallelthereto. The electrode is formed with a longitudinal slit 8 whichpermits the primary ion beam from an ion source 12 to pass through theelectrode and strike the sensitive layer 6. The electrode is maintainedat a positive potential relative to the layer 6, for example 300 volts,from a source 13.

With this modified construction ions, photons, and neutral particlesemitted from the layer 6 are intercepted by the electrode and areprevented from striking the top surfaces of the poles 1, 2. Also thepotential applied to the electrode is chosen so that the electrodeattracts secondary electrons emitted from the layer 6 and tertiaryelectrons emitted from the electrode when the ions, photons and theneutral particles strike its surface. The electrode also attracts anytertiary electrons which are emitted from the surfaces of the poles byany particles striking these surfaces.

With collecting apparatus in accordance with the invention secondary andtertiary particles are substantially prevented from striking the surfaceof the sensitive layer and therefore the sensitivity of the apparatus isnot reduced by the formation of additional marks on the sensitive layer.

snaaeaa 9 In the arrangement illustrated in the figures, the plate 5 isfixed in position. In an alternative arrangement the plate is not heldrigidly by the supports 11 so that the plate can be moved transverselyrelative to the path of the ion beam and ditferent portions of thesensitive layer" 6 can beexposed to the ion beam.

What I claim is:

1. A mass spectrometer ion collecting apparatus comprising a collectormember including a layer of photosensitive material that is sensitive toions, an electrode extending over said sensitive layer and spacedtherefrom, means for applying to said electrode a potential positivewith respect to the potential of said sensitive layer so that extendingover said sensitive layer and spaced therefrom and'formed with a surfaceextending substantially parallel to said layer, means for applying tosaid electrode a potential positive with respect to the potential ofsaid sensitive layer so that said electrode attracts negatively chargedsecondary particles emitted from the sensitive layer when primary ionsstrike said sensitive layer, and surfaces to said electrode defining aslit extending through said electrode to allow the passage of primaryions to said sensitive layer.

References Cited by the Examiner UNITED STATES PATENTS 2,489,344 11/49Washburn 250-419 2,993,992 7/61 Craig 250--41.9

FOREIGN PATENTS 759,917 10/56 Great Britain.

20 RALPH G. NILSON, Primary Examiner.

1. A MASS SPECTROMETER ELECTROSTATIC ANALYSER COMPRISING A COLLECTOR MEMBER INCLUDING A LAYER OF PHOTOSENSITIVE MATERIAL THAT IS SENSITIVE TO IONS, AN ELECTRODE EXTENDING OVER SAID SENSITIVE LAYER AND SPACED THEREFROM, MEANS FOR APPLYING TO SAID ELECTRODE A POTENTIAL POSITIVE WITH RESPECT TO THE POTENTIAL OF SAID SENSITIVE LAYER SO THAT SAID ELECTRODE ATTRACTS NEGATIVELY CHARGED SECONDARY PARTICLES EMITTED FROM THE SENSITIVE LAYER WHEN PRIMARY IONS STRIKE SAID SENSITIVE LAYER, AND SURFACES TO SAID ELECTRODES DEFINING A SLIT EXTENDING THROUGH SAID ELECTRODE TO ALLOW THE PASSAGE OF PRIMARY IONS TO SAID SENSITIVE LAYER. 